Here's how I'd look at it - to store a few items and a small bike, you don't need a very big deck (only two or three feet long at most.) Make sure that the back of your deck has a pair of lights on it so people can see where your bus ends in the dark (and that it sticks out further then the factory tail lights.) A cheap set of trailer lights should do the trick. I woldn't worry about the size being too big - I think you'll hit the weight limit of the hitch first.
This is from the Drawtite hitch website :
- Class III hitches used as weight carrying are rated up to 6000 lbs. gross trailer weight (GTW) with a maximum trailer tongue weight (TW) of 600 lbs.
- Class III hitches used for weight distributing are rated up to 10,000 lbs. gross trailer weight (GTW) with a maximum trailer tongue weight (TW) of 1000 lbs.
You're not towing a trailer, so we're interested in the tongue weight rating - it doesn't sound like you've got weight distribution hardware, so I'd say the weight limit of your hitch is 600 LBS - so the deck, the bike, and any other cargo needs to weigh 600LBS or less.